Aircraft windows, particularly side mounted windows used in aircraft passenger cabins are today typically made of acrylics. Experience has shown that acrylics are best for such use due to their optical clarity and surface hardness together with superior weatherability and good chemical resistance. These characteristics have made acrylics the material of choice, not only for aircraft windows, but many other applications.
Current aircraft windows are manufactured from stretched acrylic. Stretched acrylic is made from a block of acrylic material which is heated and mechanically pulled in both the x and y coordinates e.g. stretched until a sheet of the proper thickness is achieved. Stretched acrylic has improved structural characteristics over cast acrylic. This stretched acrylic sheet is then used to cut out the appropriate size windows which are then further treated to shape them to the desired surface contour and their edges ground or beveled to allow for mounting to the window frame.
In aircraft operation the stretched acrylic window is exposed to the outside ambient conditions encountered by the aircraft in flight, while taxiing on the ground and while parked. These ambient conditions cover the variability in atmospheric conditions on the surface of the earth wherever aircraft may fly as well as the atmosphere up to the operational ceiling of the various aircraft which may be in excess of 36,000 feet (11.2 km).
At the same time the stretched acrylic must provide the optical characteristics desired by passengers, and maintain this clarity or transparency for a long period of time. Acrylics typically provide optical transparency equivalent to glass.
While new stretched acrylic windows have a smooth surface and are completely clear, over time, following installation in aircraft, the windows develop minute lines referred to as crazing. As the number and length of these lines increase over time, the optical transparency of the window diminishes.
Recently the aircraft industry has reported a new or different form of crazing. Windows have developed miniature star shaped imperfections referred to as star crazing. These imperfections consists of irregular spaced lines of about 1.5-15 mm in length radiating from a point within a circle of about 25 mm diameter. Although those skilled in the art have established that longitudinal crazing is primarily due to flexural stresses, the exact causes of the star crazing is yet under investigation. The current consensus is that star crazing is believed to be due to increased presence of toxic or reactive chemicals in the upper atmosphere, due both to the recent rash of large volcanic eruptions and to continuing increases in man-made pollutants. These chemicals have dispersed throughout the globe, but particularly in the northern temperate zone, where not coincidentally most of the commercial aircraft operate. The chemicals react with moisture which has penetrated microscopic holes in the acrylic resulting in the irregular shaped star crazing.
The major design criteria for aircraft window design thus are, optical transparency, structural integrity, aerodynamic shape and light weight. Economics demands that the cost of the product should be as low as possible and the service life should be as long as possible. The design must balance all of these sometime conflicting requirements.
Windows made of stretched acrylic lose their optical transparency over time due to the build up of crazing as described above. Crazing is caused by moisture penetrating microscopic pinholes and/or cracks that develop due either to pressure stresses to the aircraft and due to impact at high velocity by microparticles in the atmosphere including rain and other forms of precipitation. Any impact that opens up a moisture path through a microcrack is potentially damaging.
The longitudinal crazing due to flexural stresses of the window has fortunately been a fairly slow degradation of the optical transparency of stretched acrylic windows. The star crazing observed in the past few years has unfortunately caused a much more rapid degradation. There has therefore been a need to provide a means to solve the inherent deficiency in the prior art to resist the degradation from crazing of stretched acrylic windows.
Coatings to improve the abrasion resistance of acrylics have been described, but these methods cause unacceptable weight penalties e.g. glass coating or if plastic the coatings themselves tend to crack due to the ambient flexural stresses encountered in aircraft use. U.S. Pat. Nos. 4,308,317 and 4,477,528 describe such compositions.
The current invention overcomes the prior art short comings particularly with respect to the problem of star crazing.